1. Field of the Invention
The present invention relates to a dielectric filter and dielectric duplexer used in the microwave band, etc., and to a communication device provided with the dielectric filter or the dielectric duplexer.
2. Description of the Related Art
Up to now, dielectric filters in which internal-conductor-formed holes are provided inside a dielectric block and an external conductor is formed on the outside surface of the dielectric block and which is composed of one stage or a plurality of stages of resonators have been for filters in the microwave band, etc.
In a dielectric filter using such a dielectric block, input-output terminals, which are capacitively coupled to internal conductors, are provided on the outside surface of the dielectric block for inputting and outputting unbalanced signals. Therefore, for example, in order to supply signals to a balanced-input amplifier, unbalanced signals are converted to balanced signals by using a balun (unbalanced-to-balanced converter). However, in such a construction, there are problems in that the insertion loss due to the balun is large and, since a space for the balun is required on the circuit board, the filter cannot be made smaller.
The applicant of the present invention has filed Japanese Patent Application No. 11-314657 and Japanese Patent Application No. 2000-036302 for dielectric filters in which balanced signals can be input and output without any outside help.
In related dielectric filters having balanced input-output terminals, since the axial length of each resonator is long, the required mounting area on a mounting board is inevitably increased. Furthermore, since the balanced input-output terminals are disposed in the vicinity of both open ends of a half wavelength resonator, there is a problem in that the design freedom of the space between balanced input-output terminals is low.
Accordingly, it is an object of the present invention to provide a dielectric filter and dielectric duplexer in which the above problems are solved and the freedom for arranging balanced input-output terminals is increased and which are small as a whole. It is also an object of the present invention to provide a communication device using the dielectric filter or the dielectric duplexer.
According to the present invention, a dielectric filter comprises a dielectric block; an array of three excitation holes provided inside the dielectric block, one opening of each hole being a short-circuited end and the other opening or the vicinity of the other opening being an open-circuited end, the three excitation holes being interdigitally coupled in order along the array; balanced input-output terminals provided at the open-circuited ends of the excitation holes at both ends of the array of three excitation holes; and a resonator hole which is coupled to at least one of the excitation holes at an end of the array of three excitation holes and in which one opening forms a short-circuited end and the other opening or the vicinity of the other opening forms an open-circuited end, the resonator hole being provided inside the dielectric block.
In this way, the array of three excitation holes and the resonator hole coupled to at least one of the excitation holes at an end of the three excitation holes have an axial length substantially equal to a quarter wavelength and, as a result, the dielectric filter can be made smaller as a whole. Furthermore, since the above balanced input-output terminals are provided in the open-circuited ends of the excitation hole at either end of the array of three excitation holes, the terminals can be disposed in a relatively small area.
Furthermore, according to the present invention, a dielectric duplexer comprises a first dielectric filter which is a dielectric filter having the above construction; a second dielectric filter containing another resonator hole which is different from the resonator hole of the first dielectric filter, the second dielectric filter being formed in the dielectric block having the first dielectric filter; a common input-output terminal coupled to the first and second dielectric filters, the common input-output terminal being formed on the dielectric block; and an input-output terminal coupled to the second dielectric filter, the input-output terminal being formed on the dielectric block.
Because of this construction, the dielectric duplexer can be used as an antenna-sharing device, for example, in which a filter to input or output a signal through the balanced input-output terminal is provided.
Furthermore, according to the present invention, in a dielectric duplexer, the input-output terminal coupled to the second dielectric filter and the common input-output terminal are unbalanced input-output terminals.
Because of the construction, the dielectric duplexer can be used as an antenna-sharing device, for example, in which a first filter to input or output a signal through the balanced input-output terminals and a second filter to output or input a signal through an unbalanced input-output terminal are provided and an unbalanced signal is input and output through a common input-output terminal.
Furthermore, according to the present invention, in a dielectric duplexer, the common input-output terminal is the balanced input-output terminals having the above construction.
Because of the construction, a balanced input-output type antenna can be directly used.
In an aspect of the present invention, a communication device comprises the above dielectric filter or the above dielectric duplexer.